1. Field of the Invention
This invention relates to improvements in a method for producing a sintered silicon carbide and silicon nitride base composite which is used as a material for mechanical parts (such as automotive engine parts) requiring a high strength in a high temperature, a high heat resistance and a high corrosion resistance.
2. Description of the Prior Art
A variety of methods for producing a sintered silicon carbide (SiC) and silicon nitride (Si.sub.3 N.sub.4) base composite have been proposed. One of them is disclosed in Japanese Patent Provisional Publication No. 61-36176, in which the sintered composite includes 10 to 72% by volume of SiC, 5 to 65% by volume of Si.sub.3 N.sub.4 and 5 to 40% by volume of Si. Another one is disclosed in Japanese Patent Provisional Publication 60-46973, in which the sintered composite includes SiC and Si.sub.3 N.sub.4 as major components and 0.05 to 50% by weight of rare earth element oxide. A further method is disclosed in Japanese Patent Provisional Publication No. 58-91070, in which the sintered composite is produced by mixing and sintering metals in group II, III and IV of a periodic table of the elements and oxide and/or carbide thereof with mixture powder of SiC and Si.sub.3 N.sub.4.
Additionally, the mechanical characteristics of a sintered silicon carbide and silicon nitride base composite is discussed in Journal of American Ceramic Society 56 (9) 445 (1973) and in the same 63 (9-10) 597 (1980).
Furthermore, Japanese Patent Provisional Publication No. 1-275470 discloses a sintered silicon carbide and silicon nitride base composite which is produced by hot-pressing mixture powder of silicon carbide and silicon nitride which is formed from organic silicon compounds under a gas phase method. Japanese Patent Provisional Publication No. 64-9872 discloses a sintered silicon carbide and silicon nitride base composite produced by an ordinary pressure sintering, in which silicon nitride is in needle form and of .beta.-phase, while silicon carbide is of single crystal and has a particle size of 2 to 30 .mu.m.
However, in many of such conventional methods of producing the sintered silicon carbide and silicon nitride base composite, it is necessary to employ hot pressing to promote formation of fine and dense structure of the sintered composite during sintering. Accordingly, it is difficult to produce mechanical parts having a complicated shape. Additionally, in the last-described sintered composite producing method in connection with the Publication No. 64-9872, the ordinary pressure sintering is employed and therefore the strength of the resultant sintered composite is not so improved while being high in toughness, making possible to produce mechanical parts of a complicated shape.